These experiments seek to elucidate the relationship between sequence organization of integrated feline retrovirus genes and their biological expression. Although domestic cat cells contain multiple copies of sequences related to RD114 and FeLV, these are expressed rarely as mature virus in the case of RD114, and probably not at all in the case of FeLV, without horizontal transmision. The cellular DNA's are infective only when derived from virus-productive cells. Heterologous hosts, infected with the same cat viruses are highly virus productive, but contain fewer integrated viral sequences, and the cellular DNA is generally infective by transfection techniques. Restriction mapping of unintegrated provirus, and of cell DNA's containing integrated sequences has shown dfferences in sequence arrangement between endogenous and heterologous hosts, both for RD114 and FeLV. Cloning of these sequences by recombinant DNA techniques from virus productive and non-productive cells allows us to test individual infectivities, to deduce individual restriction maps to try to correlate biological activity with sequence organization for ultimate understanding of the control of viral gene expression in eukaryotic cells. The Herpes simplex virus tk gene may be utilized for some experiments as a selectable marker, allowing us to follow the integration and expression of a retrovirus genome.